To change the clocks, edit the PLL settings in the configuration settings <code>Customize Kernel Settings</code> → <code>Blackfin Processor Options</code> → <code>Clock Settings</code> → <code>Re-program Clocks while Kernel boots</code>. The way the PLL works on the Blackfin is by taking the external crystal frequency, multiplying that up by <code>VCO Multiplier</code> (If you have a 25MHz clock, and set the Multipler to be 24, the VCO will be running at 600MHz). The core speed can be run slower than the VCO by programming the <code>Core Clock Divider</code> (Normally 1), and the <acronym title="System Clock">SCLK</acronym> must be run slower than the VCO, so it has it's own divisor, <code>System Clock Divider</code>. (If the VCO is running at 600MHz, a system clock divider of 5 will run things at 120MHz). Depending on the software load, it might be better to optimize things to have the fastest system clock possible, rather than the faster core clock.

While you can change the clocks in U-Boot by editing <code>CONFIG_<acronym title="System Clock">SCLK</acronym>_DIV</code> in <code>./include/configs/<board_name>.h</code>, it is not recommended, since you can quickly turn your board into a doorstop. Changing clocks in the kernel is the recommended way.